Electronic components such as ICs, LSIs and super LSIs using semiconductor materials such as single crystal silicon as raw material, are produced as follows: a single crystal ingot of silicon or the other compound semiconductors is sliced into thin disc wafers; a number of fine electronic circuits are built in the wafer; and then the wafer is broken up into small platelets of semiconductor element chips. The wafer that is produced by slicing the ingot is processed into a mirror surface wafer with a mirror-polished surface and edge through the steps of lapping, etching, and further polishing. After that, in a device-manufacturing step, fine electronic circuits are formed on the surface of the mirror-polished wafer.
Accompanied by development of highly integrated and high-performance LSIs, wiring technology has been shifting to new wiring structure and high integration. As the wiring becomes multilayer, an interlayer insulation film between wirings becomes also multiple layers, thereby the resulting wafer has uneven surface. Further, the uneven surface causes reduction of accuracy in processing the interlayer insulation film and wiring formed thereon, which brings about reduced reliability on LSI as well. Therefore, during a step of forming a device, a chemical mechanical polishing method (called CMP) is frequently used for planarizing a wiring metal and an interlayer insulation film in LSI production process. On the other hand, from the viewpoint of developing high-speed LSIs, the process for forming the electronic circuits has been shifting from a conventional wiring material of Al and W, to Cu having still lower electrical resistance. And a low-k film (such as a SiOC film or the like) having lower dielectric constant than a silicon oxide film (SiO2 film) is used as an interlayer insulation film.
As for a polishing method, the following method is commonly used: a work piece is put on a surface having a polishing cloth formed of, for example, a synthetic porous resin sheet, a suede-like synthetic leather, or the like; and the work piece is rotated while a polishing composition solution called a slurry is supplied at a fixed flow, under a state of pressing the work piece against the surface. The types of abrasive grains and chemical solutions for the slurry are selected depending on the material of the work piece. As an abrasive grain, cerium oxide, zirconium oxide, alumina, or silica is used. Of those, silica is frequently used for final polishing to attain an excellent mirror-surface.
As a dispersion solution for silica abrasive grains, a dispersion solution of fumed silica, colloidal silica, and the like are exemplified. The specification of the present invention refers to all the dispersion solutions as “silica slurry”.
Many technologies for planarizing a metal film and an interlayer insulation film have been developed. When those technologies are applied to planarization of a wiring metal which is finely-structured and multilayered by formation of a metal connection (plug) between the upper and lower of multilayered wirings, formation of buried wirings and the like, scratches and dishing occur in the case of broad wiring, and erosion and the like occur in the case of fine wiring. As those occurrences damage planarization, there is a concern that connection between wirings may become bad.
Of those, since a polishing method using acidic slurry can allow high polishing rate, the method can make the selectivity of a SiO2 film and a SiOC film larger. However, when acidic silica slurry is used, scratches by abrasive grains are generated. The reason for the generation of scratches can be explained as follows.
Almost all silica abrasive grain particles containing a very small amount of impurities such as aluminum oxide and the like have an isoelectric point of surface electric charge of around pH 2 to 4. The silica abrasive grain particles are negatively charged in the alkaline range and exhibit stable dispersibility, but the particles are weakly charged in the acidic range or the neutral range having a pH of 3 to 7. Therefore, dispersion stability is not maintained, thereby resulting in aggregation and gelation of the particles.
Due to the above reason, a commercially available silica slurry is adjusted to alkaline range by an alkaline component. Therefore, it is known that when the pH of the silica slurry is adjusted to acidity by adding an acid and used in the acidic range, dispersion becomes unstable, and occur aggregation or precipitation. As a result, when polishing is performed using a silica slurry containing the aggregated silica particles, scratches are generated on the wiring metal and interlayer insulation film.
On the contrary, as a method of improving dispersibility under acidic conditions, JP 2001-332518 A discloses a method of changing the surface property of silica particles by using Al. However, although the method improves dispersibility of the silica particles, the method contributes to inclusion of metals other than silicone. Metals which require caution include Al, Fe, and Cu. In particular, Al is not easily removed by cleaning, so it is considered that Al inclusion leads to a high probability of wafer contamination.
WO 89/12082 describes a polishing composition for a silicon wafer including a modified trialkylsilylation. The modified colloidal silica sol has the following effects: alkyl groups prevent the condensation reaction of a silanol group, thereby preventing silica particles from bonding to each other; generation of hard and large gel particles is suppressed; and a source of generating scratches is eliminated. Although the modified colloidal silica sol has the same object as the present invention, this silica remains to be negatively charged, because trialkylsilylation does not cause change of electric charge. Therefore, this silica can be used for polishing in an alkaline condition, but have a problem in an acidic condition.
In addition, JP 08-45934 A discloses a polishing method using silica particles whose surface property is modified by using an organic silicone compound having amino group in order to solve the above-mentioned problems. JP 08-45934 A describes that, in the method, stability of a particle dispersion is enhanced in the acidic range by negatively charging surface electric potential of the silica particles owing to surface treatment. That is, as the surface treatment reaction is carried out at a high temperature such as 145° C. in an organic solvent such as ortho-xylene or the like, the reaction will probably be different from the reaction in the present invention. For example, the reaction in the present invention is a dehydration condensation reaction of a methoxy group of γ-aminopropyltrimethoxysilane with a silanol group on the surface of silica in the presence of an acidic catalyst. Further, the method of JP08-45934A requires a reaction apparatus, a collection apparatus, and the like to collect an organic solvent, which leads not only to a production cost increase, but also to inefficient large-scale production.
JP 2003-243340 A describes a polishing slurry for a silicon wafer including silica as a abrasive grain, and organosilane containing an amino group or a partial hydrolysis condensation product thereof. JP 2003-243340 A describes that colloidal silica having a pH of 8 to 12.5 with addition of γ-aminopropyltrimethoxysilane is preferably used for polishing. In the method, γ-aminopropyltrimethoxysilane is used as an alkaline agent for the same purpose as aminoalcohol, so the surface of silica is not modified due to alkaline property thereof. Therefore, the colloidal silica of JP 2003-243340 A is different from the positively-charged colloidal silica of the present invention.
JP 2006-196508 A describes silica particles having a zeta potential of 5 mV or more measured by an electrophoresis method, an oxidant, a protective film forming agent with respect to a metal surface, and a CMP polishing solution for a semiconductor metal film containing an acid and water. The examples of JP 2006-196508 A describes use of tetraethoxysilane hydrolyzed with ammonia, benzotriazole, a silane coupling agent, and salicylic acid as an acid. The kinds of silane coupling agent are not described, and an acid is described as an etching agent for copper wiring. That is, hydrolyzation of the coupling agent is not described at all. In a condition of such a strong acid (low pKa value) as salicylic acid (pKa: 2.96), hydrolyzation of a silane coupling agent is performed at an excessively rapid rate, so salicylic acid is not suitable for modification of surface properties of silica particles. Accordingly, the electric charge value described in the examples is small.
Since a compound containing an amino group can contribute to both an effect as an alkaline agent and an effect as a chelating agent, many polishing methods using a compound containing an amino group have been proposed in documents other than JP 2003-243340 A (for example, JP 08-83780 A).
It is an object of the present invention to provide a polishing composition for a mirror-polishing process on a wiring metal, which can provide a high polishing rate in polishing a finely-structured and multilayered wiring without causing flaws called scratches on the wiring metal and an interlayer insulation film, and can provide a favorable surface roughness, and a method of producing the polishing composition. Another object of the present invention is to provide a method of producing the polishing composition in which the polishing composition is synthesized without being subjected to a high temperature heat treatment. Still another object of the present invention is to provide a polishing method for a semiconductor wafer that uses the above-mentioned polishing composition.